1. Field of the Invention
The present invention relates to a printed-circuit board for high-speed communication in which open pins of a multi-pin connector are terminated in order to eliminate influence of the open pins.
2. Description of the Related Art
In connecting many signal lines between printed circuit boards, a multi-pin connector is packaged in the printed circuit boards, and the signal lines are connected through the connector. This kind of device is disclosed in, for example, Japanese Patent Publication (unexamined) No. 2001-42981 (pages 3 to 5, FIG. 4). In a case where the number of the connector pins is larger than the number of the signal lines in the printed circuit boards, some of the connector pins are left as open pins where the signal lines are not connected. Hitherto, these open pins are left open or connected to the ground.
FIG. 13 is a graph showing a transmission characteristic of a signal line of a conventional printed-circuit board, and is a graph showing a transmission characteristic of a differential signal line connected to a connector pin.
FIG. 13 shows that transmission loss is small and signals are transmitted favorably in a low-frequency range. In a high-frequency range, the transmission loss increases at certain frequencies, and it is not possible to transmit signals in this frequency band. The transmission loss increases at frequencies higher than 3 GHz, and spike transmission loss is generated in this frequency band. Hitherto, the transmission characteristic at such a high frequency has not been examined, and the phenomenon of generation of spike transmission loss and the fact that the transmission loss is caused by the open pins connected to ground has not been known. The mechanism of this transmission loss is explained below. Both ends of each open pin are connected to the ground; therefore the connector pin operates as a resonator when the connector pin has a length that is an integer multiple of a half-wavelength of the signal. The connector pins are normally approximately 20 mm in length, and the resonance frequency is approximately 3 GHz in consideration of the relative dielectric constant of the materials such as plastic, supporting the connector pins. The open pins are joined to the signal pins connected to the signal lines; therefore energy of the signals flowing through the signal pins is absorbed by the open pins at the resonance frequency, and the transmission loss of the signal lines increases. Even if the open pins are left open, the transmission loss is increased by the same mechanism when the connector pins resonate.
Japanese Patent Publication (unexamined) No. 2001-42981 discloses an example where a connector composing a final stage of a signal line is provided with a termination board where a terminating resistance is packaged in order to reduce noises reflected from the final stage of the signal line. However, this construction has a problem that it is not possible to eliminate the influence of the open pins and prevent generation of spike transmission loss in the high-frequency band higher than 3 GHz.